Nullable types and the ternary operator: why is `? 10 : null` forbidden?
Try this:
int? x = GetBoolValue() ? 10 : (int?)null;
Basically what is happening is that conditional operator is unable to determine the "return type" of the expression. Since the compiler implictitly decides that 10
is an int
it then decides that the return type of this expression shall be an int
as well. Since an int
cannot be null
(the third operand of the conditional operator) it complains.
By casting the null
to a Nullable<int>
we are telling the compiler explicitly that the return type of this expression shall be a Nullable<int>
. You could have just as easily casted the 10
to int?
as well and had the same effect.
The compiler first tries to evaluate the right-hand expression:
GetBoolValue() ? 10 : null
The 10
is an int
literal (not int?
) and null
is, well, null
. There's no implicit conversion between those two hence the error message.
If you change the right-hand expression to one of the following then it compiles because there is an implicit conversion between int?
and null
(#1) and between int
and int?
(#2, #3).
GetBoolValue() ? (int?)10 : null // #1
GetBoolValue() ? 10 : (int?)null // #2
GetBoolValue() ? 10 : default(int?) // #3